At present it has become a business demand of more and more service providers to enable a Triple-Play network (data, voice and video). The video service is an important component of the Triple-Play network. The service providers wish to provide the video service over Digital Subscriber Line (DSL), Passive Optical Network (PON), or a wireless access such as Wireless LAN (WLAN) and a microwave access such as World Interoperability for Microwave Access (WiMAX).
Since the amount of information contained in video is large, it is almost impossible to transmit an uncompressed raw video stream via a network, which then leads to an introduction of a video coding technique. At present, the frequently used video coding techniques include MPEG2, H.264, VC-1, MPEG4, etc. An MPEG video stream includes three types of frames: I-frame, P-frame, and B-frame. Each frame is of different significance. The I-frame is a key frame of primary significance, the loss of which will make it impossible for the following P-frame and B-frame to be normally decoded, having severe influence on the video quality. The P-frame is a key frame of secondary significance, the loss of which will have influence on the decoding of the B-frame and the following P-frame, and often bring the visible distortion of video. The B-frame is a non-key frame, the loss of which will have influence only on itself and very limitedly on the video quality. When a relatively small number of B-frames are lost, the influence on the video may be negligible. It can be seen that the introduction of the video coding technique raises stricter requirements on the transmission quality of the video stream: as to the video on which the inter-frame compression is not performed, one packet lost may have influence on quality of merely one frame and the influence is limited; but as to the video stream on which the inter-frame compression is performed, a packet lost, if having influence on a key frame, may further have influence on the decoding of a plurality of the following frames and have severe influence on the video quality. Consequently, the video stream on which video coding is performed is highly sensitive to the packet lost, and a very low IP Packet Lost Rate (IPLR) must be maintained for the video service. Also, the Bit Error Ratio (BER) of a transmission line is closely related to the IPLR. A higher BER of the line will result in a higher IPLR, which may fail to meet the requirement of the video service on the IPLR.
In light of the above, a prior art solution is to only provide a video stream with a relatively low bandwidth, such as a video stream with lower quality, or to increase the compression ratio when the video stream is being compressed, based on the fact that the video stream with a lower bandwidth is relatively less sensitive to the IPLR than the video stream with a higher bandwidth. However, such a solution can not provide video programs of high quality, such as High-definition Digital Television (HDTV), and increasing the compression ratio usually needs to increase the interval of I-frames and thus results in increase in terminal decode delay, and may also enlarge the influence of one packet lost.
Another prior art solution is to provide the video service only on a DSL line with high quality and short distance, since the DSL line with high quality and short distance has a lower BER and a lower IPLR, and thus easily satisfying the requirements of the video service. However, this solution can not provide the video service for a wide range of subscribers and the video service can only serve subscribers of the DSL line with high quality and short distance, but not a great number of other subscribers.
As an improvement, there is a third solution in which a mechanism of end-to-end application layer Forward Error Correction (FEC) is employed in the video service. The so-called mechanism of application layer FEC lies in that FEC coding is employed at the application layer to add redundant error correction information. Thus, even if an error occurs in transmission that can not be corrected at the network layer, error correction may be performed at the receiving terminal through this application layer FEC coding redundancy information. In this solution, the video stream sent from a video source includes not only an ordinary video stream on which video coding has been performed, but also an FEC stream formed by an ordinary video stream through FEC coding. Thus a data stream with redundancy information can be provided for the receiving terminal, and the lost and damaged video stream may be recovered to some extent by means of the FEC. Specifically, this solution includes the following steps.
A1. At a video source, FEC coding is performed on a video stream on which video coding has been performed to generate an FEC stream.
For the same FEC coding, the greater the overhead of the FEC coding is, the better the error correction ability is. In other words, to reach the same target IPLR, the higher the IPLR without the FEC is, the greater the needed overhead of the FEC coding is.
A2. The raw video stream and the FEC stream is transmitted.
The transmitted raw video stream and the FEC stream are to be transmitted to a user terminal via a core network and an access network line, such as a DSL line.
A3. The user terminal receives the raw video stream and the FEC stream, and recovers the damaged data in the raw video stream according to the FEC data.
However, the above method has the following drawbacks.
1. The bandwidth of the network is wasted.
In general, the Quality of Service (QoS) of the core network can satisfy the requirements of the video service and the application layer FEC is not required. The FEC is only required in the access network. Thus such an end-to-end FEC actually wastes the bandwidth of the network.
2. The method can not be adapted to different access lines.
The conditions of access networks are different from one another. The BER of different access lines may differ greatly due to the quality, length and electromagnetic environment of the lines. There may be even a coexistence of different access techniques, such as the coexistence of DSL and WLAN. However, if an end-to-end method is used, the worst condition has to be considered to meet the requirements of all the lines. It is thus required that the video source employ the FEC with a maximum overhead. In fact, only a very small number of lines need the maximum overhead, and more lines just need less overhead. Many lines even do not need the application layer FEC at all, since these lines may meet the requirements by themselves. In other words, as to certain lines, this method wastes a large amount of bandwidth of the network and the line.
3. It is necessary for the video source to support the FEC function that most of the existing video sources do not support, resulting in limiting of the application of the video service.
4. Since the subscriber line is influenced by many external factors, its BER may vary within some range, which makes it difficult to determine how much the overhead of the FEC is needed to meet the requirements of all the subscribers.